It is the broad objective of this investigation to determine the mechanisms by which fatty acid and complex lipid biosyntheses are regulated in animal tissues. Since acetyl CoA carboxylase is the primary regulatory enzyme in these processes, the investigation of its enzymatic carboxylation mechanism and the mechanism of regulation of its catalytic activity is of fundamental importance. Both the liver and adipose tissue carboxylases have been obtained in pure form in our laboratory and have already been studied in some detail. Activation of these carboxylases by the allosteric effector, citrate, promotes the transition of the protomeric form to a polymeric filamentous form of the enzyme (4-8,000,000 molecular weight). Preliminary evidence indicates that citrate activation of the carboxylase increases the reactivity of the vitamin (plus minus biotin) prosthetic group at the active site of the enzyme. The Specific Aims of this investigation are: 1) to determine the mechanism of citrate activation of acetyl CoA carboxylase, 2) to determine the mechanism by which the biotin-containing carboxylases catalyze CO2 fixation (with particular reference to the acetyl CoA carboxylases), 3) to characterize the liver and adipose tissue acetyl CoA carboxylases with respect to subunit and quaternary structure, 4) to determine the citrate-induced alterations in physical- chemical properties of the protomeric form of the carboxylase which accompany increased catalytic activity and polymerization, 5) to determine the relationship of these phenomena to the biological control of fatty acid biosynthesis, and 6) to determine the structural or organizational role of the acetyl CoA carboxylase "filaments" in lipid biosynthesis.